Some central concepts used in the following text are briefly defined here.
Laterally Propagating Plate Wave Modes
Bulk acoustic thickness vibration arising in a piezoelectric plate (a wafer or a thin film layer) can propagate laterally (in the horizontal direction) in the plate. Such laterally propagating wave modes are called plate wave modes or Lamb wave modes [1]. The term plate wave mode will be used here. The wave propagates with a lateral wavelength λx and has a lateral wave number kx.
Different types of thickness vibration can propagate as plate waves. Examples of such vibration types are the thickness-extensional (TE) and thickness-shear (TS) vibration (FIG. 1). In the former, the particle displacement is in the thickness direction of the piezoelectric plate (z-axis in FIG. 1), and in the latter, it is perpendicular to the thickness direction. A thickness resonance arises within the piezoelectric plate when the thickness t of the plate is equal to an integer number of half-wavelengths λz: t=Nλz/2, λz=v/f, where N is an integer, v is the velocity of the acoustic wave in the piezoelectric material, and f is the operation frequency. In the first-order mode, N=1 and there is one half-wavelength accommodated within the thickness t.
In FIG. 1, some plate wave types are depicted. From top to bottom: the first-order thickness-extensional mode TE1, the second-order thickness-shear mode TS2, the first-order thickness-shear mode TS1, flexural mode. Propagation is in the lateral direction with lateral wavelength λx.
Lateral Standing Wave Resonances
Laterally propagating plate waves can be reflected from discontinuities, such as electrode edges. In a laterally finite structure, therefore, lateral standing wave resonances can arise. A lateral standing wave resonance arises when the lateral dimension W of the finite structure equals an integer number of half-wavelengths λx of the lateral propagation: W=Nλx/2. The integer N implies the order of the resonance. In the first order resonance, there is one half-wavelength within the lateral length of the structure.
Lateral standing wave resonances can arise for any thickness vibration mode (e.g., TE mode or TS mode).
In FIG. 2, the first two lateral standing wave resonances in a laterally finite plate with width W and thickness t are shown for the TE1 and the TS2 thickness vibration modes (plate wave modes). The first resonances (2a and 2c) have a lateral wavelength of λx=2W are symmetric in the width of the plate while the second resonances have a lateral wavelength λx=W (2b and 2d) are antisymmetric in the width of the plate.
Dispersion Diagrams
Relation between the lateral wave number kx of a laterally propagating plate wave and the frequency f is called the dispersion of the plate wave and is presented as a dispersion diagram. In a dispersion diagram, negative x-axis often corresponds to imaginary wave number (evanescent wave), whereas positive x-axis corresponds to real wave number (propagating wave).
In FIG. 3, a calculated dispersion diagram is shown. Second-order thickness shear (TS2) and first-order thickness extensional (TE1) plate wave modes are denoted.
Electrical Coupling of Resonance Modes
Mechanical vibration in a piezoelectric film produces an electrical field. For this field to create voltage between the electrodes of a resonator, it needs to be such that the total charge over the electrode is not zero.
Acoustical Coupling
Mechanical vibration can couple mechanically from one resonator structure to another. Mechanical coupling can happen via an evanescent wave or via a propagating wave.
General Description of Related Art
Radio-frequency (RF) components, such as resonators and filters, based on microacoustics and thin-film technology are currently widely used in radio applications, such as mobile phones, wireless networks, satellite positioning, etc. Their advantages over their lumped-element counterparts include small size and mass-producibility. Two principal microacoustic technologies used for RF devices are surface acoustic wave (SAW) and bulk acoustic wave (BAW) technologies.
In this section, existing filter technologies are briefly introduced to provide background for the current invention and to distinguish the invention from the prior art.
Surface Acoustic Wave Devices
Interdigital transducers (IDTs)—comb-like structures of thin-film metal strips, see FIG. 4—are patterned on a piezoelectric substrate (e.g., quartz, LiNbO3 or LiTaO3). The IDTs are used to transform the electric input signal Vin into a surface-propagating acoustic wave via the piezoelectric effect, as well as to pick up the acoustic signal at the output port and transform it back to electrical form. The operation frequency of the device depends on the velocity of the acoustic wave and the dimensions of the IDT electrodes via f=2p/v, where f is the frequency, p is the period of the IDT, and v is the velocity of the surface wave. Therefore, higher operation frequencies require smaller p if the velocity is kept constant.
Bulk Acoustic Wave Devices
In a BAW device, acoustic vibration inside a piezoelectric wafer or a thin film is used to process the electrical input signal. In a solidly-mounted BAW resonator (SMR), an acoustic reflector composed of alternating high and low acoustic impedance (Z) material layers serves to isolate the vibration in the piezoelectric thin film from the substrate and to prevent acoustic leakage. In a membrane device the same is accomplished by fabricating and air gap between the piezoelectric resonator and the substrate.
Thickness Vibration and Plate Wave Dispersion
As explained above, in the piezoelectric layer, different thickness vibration modes, such as longitudinal (also called thickness-extensional, vibration is in the thickness direction, parallel to z-axis) and shear (vibration perpendicular to z-axis) vibration, arise as the excitation frequency f is swept. Such thickness vibration can propagate in the lateral direction as a plate wave. Acoustic properties of the plate waves can be described with dispersion curves, in which the lateral (perpendicular to z-axis) wave number kx of the plate wave is presented as a function of frequency f.
FIG. 3 shows calculated dispersion diagram for the thin-film layer stack given in Table 1. The dispersion curves of the laterally-propagating plate waves for the first-order longitudinal (thickness extensional, TE1) vibration mode, in which the thickness of the piezoelectric layer contains approximately half a wavelength λz of the thickness vibration, and for the second-order thickness shear (TS2) mode, in which the particle displacement is perpendicular to the thickness direction and one acoustic wavelength λz is contained in the piezoelectric layer thickness, are denoted in the figure. This type of dispersion, in which the onset frequency of the TE1 plate mode is higher than the onset frequency of the TS2 plate mode, is called Type 1[2]. Onset means the point at which the dispersion curve of a plate mode crosses the kx=0 axis (i.e. the frequency axis). Type 1 materials include, e.g., ZnO. Aluminum nitride is inherently Type 2 (TS2 is higher in frequency than TE1).
By designing the thin-film stack correctly, dispersion properties can be tailored and the dispersion type changed.
Table 1 is the film thicknesses used in the calculation of the dispersion curves in FIG. 2.
MaterialSiO2WSiO2WSiO2TiMoAINAlThickness790505620510950103601750220(nm)
In FIG. 3, positive values of kx denote a real wave number (propagating wave) and negative values correspond to imaginary wave number (evanescent wave).
For a lateral standing wave resonance to arise in a laterally finite structure, the acoustic energy must be trapped inside the resonator structure, both in the thickness direction and in the lateral direction. In the thickness direction, isolation from the substrate (reflector or air gap) ensures the energy trapping. In the lateral direction, there should be an evanescent wave outside the resonator region for energy trapping. Energy trapping is easier to realize in Type 1 dispersion. Therefore, when using AlN as the piezoelectric material, the reflector is usually designed so that it converts the dispersion into Type 1.
In a laterally finite plate resonator, a propagating plate wave can form lateral standing wave resonances when the width W of the resonator accommodates an integer multiple of half-wavelengths, i.e. W=Nλx/2.
Acoustical Coupling in BAW Devices
A filter can be made by electrically connecting one-port resonators to form a ladder or a lattice filter. Another possibility is to arrange mechanical (acoustic) coupling between resonators by placing them spatially close enough to each other for the acoustic wave to couple from one resonator to another. Such devices are called coupled resonator filters (CRF).
In BAW devices, vertical acoustic coupling between stacked piezoelectric layers is used in stacked crystal filters (SCF) and vertically coupled CRFs. In an SCF, two piezoelectric layers are separated by an intermediate electrode. In a vertically coupled CRF, coupling layers are used to modify the coupling strength between the piezoelectric layers. The CRF can be fabricated either using the SMR or the membrane technology.
A thin-film vertically coupled CRF has been shown to give a relatively wide-band frequency response (80 MHz at 1850 MHz center frequency, or 4.3% of center frequency). They also enable unbalanced-to-balanced (balun) conversion. The disadvantage of the vertically coupled CRFs is the need for a large number of layers and the sensitivity of the response to the thickness of the piezolayers. This makes the fabrication process difficult and consequently expensive.
Lateral acoustical coupling in BAW devices (LCRF) can be realized with 2 or more narrow electrodes placed close to each other on the piezoelectric layer. Electrical input signal into a first port formed by one or more electrodes is transformed into mechanical vibration via the piezoelectric effect. This vibration couples mechanically across the gap to a second port formed by one or more electrodes and creates an output electrical signal. Electrodes in this example are interdigital (comb-like), but other shapes are possible as well. Coupling strength is determined by the acoustic properties of the structure and by the gap between the electrodes.
Bandpass frequency response in an LCRF is typically formed by two lateral standing wave resonances arising in the structure. Typically one of the resonances forming the passband is the lowest-order (symmetric) lateral standing wave resonance for which N=1 and all electrodes are vibrating in-phase (see FIG. 7). Preferably the second resonance is the one for which every electrode is in the opposite phase with its neighbours (odd resonance mode) (see FIG. 7). The frequency difference between the lateral standing wave resonances determines the achievable bandwidth of the filter, and depends on the acoustic properties of the structure and on the electrode dimensions and the acoustical coupling strength between the electrodes.
Prior art describes LCRF structures using two lateral standing wave resonances arising for one plate wave mode, e.g., TE1, to form a bandpass response. Also, in the prior art, there has been identified unwanted spurious resonances and passbands at the frequency of other plate wave modes existing in the structure, e.g., TS2 mode. [3]
The main advantage of the LCRF over the vertical CRF is the simple fabrication technology, as only one piezoelectric layer and no coupling layers are required, as opposed to the vertical CRF. Operation at high frequencies is easier than for SAW components, as the operation frequency is mainly determined by the layer thicknesses, not the electrode dimensions, relaxing requirements for very narrow dimensions.